In recent years, there has been a rapidly growing demand for liquid crystal display devices, etc. in the form of flat panel displays. Since liquid crystal display devices consume less electric power and can be more easily made in a small size than CRTs (cathode-ray tubes), liquid crystal display devices have been widely used in cellular phones, portable game machines, in-vehicle navigation systems, etc., as well as televisions. Among these liquid crystal display devices, active-matrix liquid crystal display devices have been widely used because they are high in response speed and make it easy to display multiple tones.
However, an attempt to achieve an active-matrix liquid crystal display device or, in particular, an active-matrix liquid crystal display device having a wider display screen with higher resolution makes shadows likely to appear, thus undesirably degrading image quality.
(a) and (b) FIG. 10 are diagrams for explaining shadows appearing on the display screen of a liquid crystal display device.
For example, in such a case as that shown in (a) of FIG. 10 where the liquid crystal display device displays a screen image having a background of a certain gray scale and a window of another gray scale in the background, shadows that are different from the original gray scales may appear on the upper, lower, right, and left sides of the window. The shadows appearing on the right and left sides of the window, i.e., the shadows appearing in areas A, are called “horizontal shadows”, and the shadows appearing on the upper and lower sides of the window, i.e., the shadows appearing in areas B, are called “vertical shadows”.
Because a vertical shadow and a horizontal shadow are attributable to different causes, it is necessary to take separate measures against them.
First, the cause of a vertical shadow is explained with reference to FIG. 11.
FIG. 11 is an equivalent circuit diagram of an active-matrix liquid crystal display device. The liquid crystal display device shown in FIG. 11 includes: a plurality of scanning signal lines X1, X2, and so forth; a plurality of data signal lines Y1, Y2, and so forth orthogonal to the scanning signal lines; and a plurality of display elements P (regions surrounded by dotted lines) provided at points of intersections between the scanning signal lines and the data signal lines, respectively. Each of the display elements P corresponds to a single pixel (or a single subpixel). FIG. 11 shows a point Q that corresponds to a pixel electrode connected to the drain electrode of a switching transistor TR and to one electrode of a liquid crystal cell (liquid crystal capacitor) Cx.
The pixel electrode in each display element P forms parasitic capacitors (source-drain capacitors) Csd1 and Csd2 with two data signal lines, respectively, between which that display element P is interposed. For this reason, even when the switching transistor TR is off, a change in voltage of the data signal lines leads to a change in drain voltage (voltage at the point Q) of the switching transistor TR, so that there is also a change in liquid crystal application voltage, which is a difference between the drain voltage and a common electrode voltage Vcom. Further, the liquid crystal molecules contained in each pixel element P respond to the root-mean-square of a voltage that is applied to the liquid crystals during a single vertical period. For this reason, even when two display elements arranged in the same row are supplied with the same voltage by turning on the switching transistors TR, the two pixels differs in luminance from each other if the two data signal lines between which one of the display elements is interposed and the two data signal lines between which the other display element is interposed differ in voltage from each other while the switching transistors TR are off. For the reasons stated above, a vertical shadow appears on the display screen.
Let it be assumed here that the liquid crystal display device shown in FIG. 11 is a normally white liquid crystal display device in which dot-reversal driving is carried out, that P(i,j) denotes a display element P provided at a point of intersection between a scanning signal line Xi and a data signal Yj, and that PX(i,j) denotes a pixel corresponding to the display element. Further, for simplification of explanation, only the influence of the parasitic capacitor between the pixel electrode in the display element P(i,j) and the data signal line Yj is taken into account, and the influence of the parasitic capacitor between the pixel electrode and a data signal line Yj+1 is disregarded.
FIG. 12 is a signal waveform chart showing a voltage in the display element P(i,j) in a case where the pixel PX(i,j) is in an area C (where there is no vertical shadow) of (b) of FIG. 10.
As shown in FIG. 12, the voltage of the scanning signal line Xi is at a high level only during a single horizontal period in a single vertical period. While the voltage of the scanning signal line Xi is at a high level, the switching transistor TR is turned on, so that the drain voltage of the switching transistor TR becomes equal to the voltage of the data signal line Yi. After that, when the voltage of the scanning signal line Xi is changed to a low level, the switching transistor TR is turned off. Even while the switching transistor TR is off, a change in voltage of the data signal line Yi leads to a change in drain voltage of the switching transistor TR, so that there is also a change in liquid crystal application voltage.
In a conventional liquid crystal display device, a voltage corresponding to black data, for example, is supplied to the data signal line Yj during a vertical flyback period. For this reason, in the normally white liquid crystal display device, there is a great change in voltage of the data signal line Yj during a vertical flyback period, with the result that there are great changes both in drain voltage of the switching transistor TR and in liquid crystal application voltage.
The effective value Vrms of the voltage applied to the liquid crystals in the display element P(i,j) is equal to the root-mean-square of the voltage applied to the liquid crystals during a single vertical period, as represented by expression (1) as follows:Vrms={(∫{f(t)}2dt)/T}1/2  (1),where f(t) is the liquid crystal application voltage and T is the period of time from the completion of writing of data to a display element P to the start of next writing of data to the same display element P (as obtained by subtracting a single horizontal period from a single vertical period).
FIG. 13 is a signal waveform chart showing a voltage in the display element P(i,j) in a case where the pixel PX(i,j) is in an area B (where there is a vertical shadow) of (b) of FIG. 10. In FIG. 13, albeit similar to FIG. 12, there is a great change in voltage of the data signal line Yj during a window display period as well as a vertical flyback period, with the result that there are great changes both in drain voltage of the switching transistor TR and in liquid crystal application voltage.
A contrast between FIG. 12 and FIG. 13 shows that a display element corresponding to a pixel in an area C and a display element corresponding to a pixel in an area B differ in effective value of liquid crystal application voltage. For this reason, the pixel in the area C and the pixel in the area B differ in luminance from each other, with the result that a vertical shadow appears.
Patent Literature 1 describes an active-matrix liquid crystal display device that prevents a vertical shadow.
The active-matrix liquid crystal display device is described below with reference to FIG. 14.
FIG. 14 is a block diagram showing a configuration of the liquid crystal display device.
As shown in FIG. 14, a display control circuit 211 includes a timing control section 212, a column data calculation section 213, a look-up table (hereinafter referred to as “LUT”) 214, a switch 215, and an LUT control section 216. The display control circuit 211 functions as a data processing circuit to obtain vertical flyback period data B in accordance with image data D inputted thereto and change between outputting the image data D and outputting the vertical flyback period data B.
Specifically, the column data calculation section 213 carries out a predetermined calculation of column-wise data contained in image data D inputted thereto, and outputs a calculation result A. The LUT 214 converts the calculation result A into vertical flyback period data B. The switch 215 switches, in accordance with a timing control signal TC, between outputting the image data D during an effective period of the image data D and outputting the vertical flyback period data B during a vertical flyback period. A data signal line driving circuit 203 drives data signal lines Y1 to Ym in accordance with the data outputted from the display control circuit 211. When the image data D is moving image data, the display control circuit 211 may stop the process of obtaining vertical flyback period data B, and may obtain vertical flyback period data B in accordance with the ambient temperature and the intensity of outside light.
The foregoing configuration makes it possible, even when a change in data signal line voltage results in a change in liquid crystal application voltage retained in a display element, to use suitable vertical flyback period data B to control to a desired level the effective value of the liquid crystal application voltage retained in the display element, thus making it possible to control the luminance of the display element and thereby prevent a vertical shadow from appearing on the display screen.